1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a folded corner of a sheet which stops upon contact with front lays at the tip of a feedboard in a sheet-fed printing press.
2. Description of the Related Art
With a sheet-fed printing press, sheets stacked on a pile board of a feeder are sucked, one by one, out of an upper part of the stack by a suction device, and fed out onto a feeder board. The sheet is further fed out onto the feedboard by a transport tape and a feeding roller. This sheet has its circumferential position registered by the front lays at the tip of the feedboard and stopped in this state, and also has its lateral position registered by a side lay device. The sheet which has stopped upon contact with the front lays is gripped and transported by grippers of a swing arm shaft pregripper, is then gripped, for gripping change, by grippers of an impression cylinder of a printing unit, and is printed while being transported.
The sheets which are thus supplied to the printing unit need to be supplied, one by one, without being distorted or folded. If an improper sheet feed such as a corner fold or a double-sheet feed happens, therefore, it is detected, and gripping by the grippers or feeding onto the feedboard is stopped. The corner-folded sheet among such improperly fed sheets is a sheet having the right corner (right end) or the left corner (left end) of its front edge (front endpart) folded in a triangular shape. To detect this folded corner, it has been common practice to provide a photoelectric detector above a position several millimeters ahead of the front lays, detect whether or not there is reflected light from the sheet stopping when contacting the front lays, and judge that the sheet is folded, in the absence of the reflected light, thereby detecting the folded corner. If the sheet size changes, the detector is moved.